The TRUTH in efficiency ratings

Don't mean much, IMHO.....It appears central is using combustion efficiency - to that, you have to figure in heat transfer efficiency and heat loss (BIG on outdoor boilers with pipe runs).......So the delivered efficiency of such a boiler into the water in the house might be as low as 40% or as high as 60-something percent.

Unfortunately, it is impossible to compare apples to apples because we don't have the type of standards in place to do so. It could be done by an independent party, but the cost would be quite high and there would be little reason for doing so.

So to answer the question - no difference at all based on advertised figures, etc....

One of the moderators in the Boiler Room is an engineer with quite a bit of experience in these matters and has measured his entire wood boiler system efficiency at about 60%....that is with a top notch high efficiency boiler, storage, tuning, good operator, etc.........

In a rough sense, wood contains about 8,000 BTUs per dry pound. If you end up getting 3500-4000 of those into your house you are in great shape!

There are a few threads on this topic. Bottom line - it's mostly smoke and mirrors. There are a few standards, but they don't generally correlate with each other or with real-world results.

The only test results that I'm familiar with are the results from a European test of the EKO 80. As I remember, it got a 91% rating based on 8600 BTU per pound of dry wood. However, that was burning REALLY dry fuel (birch chips at 15% moisture) and a flow rate of 35 gpm through the boiler at an inlet temp of around 135 degrees.

That 91% number is very close to the theoretical maximum that you can get with a non-condensing boiler, since you HAVE to lose the heat of vaporization for the water that's in the wood and the water that's a byproduct of combustion. That accounts for about 6 or 7% of the total heat generated by burning the wood, if I remember rightly. The EKO also lost 2% to the surroundings through the sides of the boiler. You could do better, but not by much.

I don't think that there's any way that anyone's getting 94% or more out of a wood boiler.

"Puffing," a term given to an occasional downdraft puff of smoke from some boilers, more aptly describes the efficiency claims of some boiler mfrs.

Starting with the efficiency number, some claims relate to "burn efficiency," which I take to mean the % of energy in the unburned wood which through combustion is released as heat. IMO a good gasification boiler should be around 97-98% in this category. Some claims relate to transfer of the heat produced during combustiion to the water circulating through the boiler. Typical % here for a good gasification boiler might fall in the range of 83-86% or even a little better. The problem with this latter claim is that it may be based on a laboratory burn, with precise control of wood moisture content, wood split sizes, loading, flue temperature, air mix and other variables, all of which a user would have great difficulty duplicating. Also, one has to consider whether the claims are made on the high heat value or low heat value of wood, and I assume they are made based on low heat value.

That said, based upon some reasonable assumptions and weighed wood burns, I have calculated with a high degree of confidence efficiency at 86% [low heat value] for my Tarm in ability to transfer to water the heat from combustion, and I have posted this on the forum. At the same time, I operate my Tarm based on assumed 80% efficiency, as it takes some care and extra time to operate it in the most efficient manner that I can. I made a similar finding of 86% for a Garn WHS3200, but that was based on a calculated flow rate which added a significant element of uncertainty, and I calculated efficiency into the low 90% for a Wood Gun E500, but that too was based on a calculated flow rate which similarly added a significant element of uncertainty. A recent post for an Effecta also found 86% efficiency.

Who can you believe? I think the best info is from users who post data-backed findings on this forum. And if there are no posts from users based on data, I would view with skepticism claims by the mfr. I might surmise, however, that similar boilers from different mfrs employing similar burn technologies may be close in efficiency. I cannot make the same assumption concerning a boiler with a different burn technology. A mfr's use of the word "gasification" is not, IMO, adequate evidence of similarity in burn technology. Burn technology is design of firebox, design of the gasification chamber geometry and materials, heat exchanger tube design, and control of the air mix during the burn to the firebox and to the gasification chamber, and more.

What efficiency will you experience? Your ability to achieve a high level of efficient operation may not equal that of some experienced users who have reported findings based on data, but at least you should have some reasonable expectation of performance you could achieve if you get most things "right" in the use of the same or similar boiler.

In the solar industry we have simulation software to show the energy movement. Here are some of the printouts from the Valentin T-sol program. I was amazed how much of the energy slips away from the collector to the final heat emitter. I would imagine the same for wood fired energy transfer.

It's not just about the wood to water efficiency, but wood to load.

hr

FWIW, here's a couple facts.

This is not common knowledge but from what I understand, all of the ultra high 90% ratings came from one particular testing lab.

The EPA has taken the efficiency numbers off their website.

A new testing format is being researched by EPA.

Any appliance doing over 90%, be it gas, oil or wood fired, will have to have a condensate drain somewhere in order to handle the moisture produced by collapsing the flue gas vapor. Have you ever seen a condensate drain on a wood burner?

....draw your own conclusions.

Have you ever seen a condensate drain on a wood burner?

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Hmmmm good point !

Will

If you look at other numbers the like of CB quote they seem to acknowledge that their OWB's are about 30% and their pseudo gassers 50%. I assume that other manufacturers similar products would be similar.

Some of the really high tech gassers may get into the 90% something, the lower tech ones 80% ish.

It may well be possible to get the two OWB variants up if they were operated in an efficient manner, but that would not be the norm.

There is a lot that could be gained from analysis of the MPG that auto mfg's were using to establish their cars fuel efficiency. The results that were assumed were from a process that was seldom if ever practical. In a global sense and in a personal application most everybody knew the smoke and mirrors were the general rule of thumb for the numbers that the public was being encouraged to swallow. In this forum though we get to hear real results from different approaches by different users and it is a real plus.
For myself I use less labor to supply the cash I need to pay for heat and hot water I need for my domestic use. If I have to use the standard ecnomy of "working for the boss" and doing overtime to accumulate the funds needed to pay for the overpriced fossil fuels I find a chart that is easy for me to read.(i.e.: it costs me four+ times as much to heat my home and supply my domestic hot water by standard economy compared to buying and splittng and burning wood. Hence it is easy to see my boiler is about 400% efficient.)

Cave2k said:

For myself I use less labor to supply the cash I need to pay for heat and hot water I need for my domestic use. If I have to use the standard economy of "working for the boss" and doing overtime to accumulate the funds needed to pay for the overpriced fossil fuels I find a chart that is easy for me to read.(i.e.: it costs me four+ times as much to heat my home and supply my domestic hot water by standard economy compared to buying and splitting and burning wood. Hence it is easy to see my boiler is about 400% efficient.)

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I figure closer to 450% as I don't have to pay for a gym membership to get my lazy butt off the couch and do some hard, btu producing work

Cave2k said:

There is a lot that could be gained from analysis of the MPG that auto mfg's were using to establish their cars fuel efficiency. The results that were assumed were from a process that was seldom if ever practical.

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I got a kick out of the one where one of the diesel engine manufacturers here in the States got busted for an Easter Egg in their engine management firmware that would recognize when the engine was undergoing EU compliance testing and would transition to a special mode designed solely to pass the test. One of those cases where you can't help admiring the bad guys a little.

Willman said:

Have you ever seen a condensate drain on a wood burner?

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Hmmmm good point !

Will

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Yup, it was right out the front door of the Cyclone Seperator on our woodgun back in the 80s. Black water would just ooze right out and onto the floor.

ewdudley said:

Cave2k said:

There is a lot that could be gained from analysis of the MPG that auto mfg's were using to establish their cars fuel efficiency. The results that were assumed were from a process that was seldom if ever practical.

Click to expand...

I got a kick out of the one where one of the diesel engine manufacturers here in the States got busted for an Easter Egg in their engine management firmware that would recognize when the engine was undergoing EU compliance testing and would transition to a special mode designed solely to pass the test. One of those cases where you can't help admiring the bad guys a little.

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Hmmm... True, but with that much ingenuity you'd think they could do it right and realize it's a better way to be at the top of the line. Interesting to some and inneressing to others.

sdrobertson said:

Cave2k said:

For myself I use less labor to supply the cash I need to pay for heat and hot water I need for my domestic use. If I have to use the standard economy of "working for the boss" and doing overtime to accumulate the funds needed to pay for the overpriced fossil fuels I find a chart that is easy for me to read.(i.e.: it costs me four+ times as much to heat my home and supply my domestic hot water by standard economy compared to buying and splitting and burning wood. Hence it is easy to see my boiler is about 400% efficient.)

Click to expand...

I figure closer to 450% as I don't have to pay for a gym membership to get my lazy butt off the couch and do some hard, btu producing work

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I like that +1